In recent years, along with the influence of a decrease in liquid crystal panel prices and the start of the digital terrestrial broadcasting service, flat panel displays have rapidly spread. Of flat panel displays that are currently in practical use, liquid crystal displays are widely used in not only TV or PC monitors, but also in mobile devices such as cellular phones, portable game devices and tablet PCs, and the liquid crystal panel market is expanding now.
Color filters function to colorize liquid displays. Under the above circumstance, there is an increasing demand for color filters with higher luminance, higher contrast and improved color reproducibility. Recently, due to reduction in power consumption of backlights and properties of LED backlights, there is especially a growing demand for color filters with higher luminance.
The above-mentioned issues apply to organic EL displays, which are expected to widely spread in the future, and higher luminance and improved color reproducibility are also the issues organic EL displays have to solve.
In general, a resin composition for forming a color filter is used to form color layers of color filters, the composition containing a pigment or dye as the color material. Compared to dies, pigments have better resistance properties, such as high heat resistance. However, sometimes color filters produced with pigments have insufficient luminance.
On the other hand, when dyes are used as the color material, color filters with high luminance can be obtained. However, the color filters have a problem with the resistance properties, such as heat resistance and light resistance.
As a method of increasing the resistance properties of dyes, a method of forming a salt-forming compound has been known (for example, see Patent Literatures 1 to 4).
An example is disclosed in Patent Literature 1, in which a chloride ion or an aryl sulfate ion is used as a counter anion of a triarylmethane dye to improve heat durability of the triarylmethane dye.
In Patent Literature 2, as a method of obtaining a coloring resin composition which has not only excellent light resistance but also excellent heat resistance, a salt forming method has been reported, in which a salt is formed by using a sulfonated compound of a dye skeleton such as phthalocyanine or anthraquinone, which is the counter anion, in combination with a triarylmethane skeleton, which is the cation.
However, the salt-forming compounds described in Patent Literatures 1 and 2 are those which are basically dissolved in solvents for use. They have insufficient heat resistance and light resistance.
In Patent Literature 3, as a method of obtaining a color composition for color filters with excellent color characteristics, heat resistance, light resistance and solvent resistance, a salt-forming compound comprising a triarylmethane basic dye and a sulfonated organic compound having at least two sulfonic groups, is disclosed.
In Patent Literature 4, as a blue pigment for color filters that is able to realize liquid crystal display with excellent luminance for a long period of time and at high temperature, a basic triarylmethane dye in which one or more elements selected from tungsten, silicon and phosphorus, and oxygen are contained as essential elements in the anions, is disclosed.
However, as will be shown below with comparative examples, the technique of Patent Literature 3 provides poor light resistance, while the technique of Patent Literature 4 provides poor heat resistance.
A polysiloxane dye is disclosed in Patent Literature 5, which is highly cross-linked by polysiloxane containing at least ten Si atoms. Due to its synthesis method, the polysiloxane dye disclosed in Patent Literature 5 is a mixture in which an unreacted compound having only one dye skeleton or dyes with different polymerization degrees are present. It is difficult to separate only a dye with a specific polymerization degree from the polysiloxane dye, so that there is a problem with the productivity of the polysiloxane dye. Since the polysiloxane dye contains a silanol group or alkoxysilyl group, a siloxane bond is formed between the polysiloxane dye(s) or between the polysiloxane dye and other component having a silanol group or alkoxysilyl group. As a result, there is a deterioration in the state of a solution or dispersion liquid containing the polysiloxane dye, such as a change in solubility or an influence on dispersion stability, making it difficult to handle the solution or dispersion liquid. This reaction is likely to proceed especially by heating, so that the dye of Patent Literature 5 is not suitable for color filter applications. As will be shown below with comparative examples, the color filter containing the polysiloxane dye has poor heat resistance.